摘要:

Examination of some well‐preserved specimens of Silurian rugose corals from Gotland reveals that the following categories of life strategies can be distinguished: ambitopic, liberosessile, fixosessile, rhizosessile and possibly limited vagile. Most solitary rugose corals appear to have been liberosessile and are characterized by initial attachment to a small sediment grain but subsequently becoming recumbent on a soft substrate. Detailed studies ofPhaulactis angelini, Holophragma calceoloides, Laccophyllum lindstroemi, Rhegmaphyllum conulusandRhabdocyclus ocksaroensisn. sp., and comparison with earlier described species, provide some new aspects on different life strategies for solitary coral

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb02061.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Ocular structures have recently been discovered on the cephalon of an Ordovician agnostid trilobite,Oculagnostus frici(Holub, 1908). They are represented mainly by obvious palpebral lobes situated at the postero‐lateral part of the genae. In addition, in one specimen there is a dome‐shaped structure below the palpebral lobe. This may well be the remains of an eye, but details of its structure are indeterminate. In the other two cephala to hand there is a notch or cavity beneath the palpebral lobe, indicating that the visual surface may have been encompassed by a circum‐ocular suture such that the lentiferous region normally dropped out during ecdysis. The origin of the ocular structures inOculagnostus friciis briefly discussed.Oculagnostusn. gen. is desc

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb02062.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Although it is common knowledge that many trilobites enrolled, behavioral and functional aspects of enrollment are not at all well understood. Taphonomic details portrayed by enrolled trilobites in the Middle Devonian Hamilton Group (New York State) indicate that enrollment was a complex and morphologically constrained behavior. The trilobitesPhacops rana(Green) andGreenops boothi(Green) are frequently enrolled in Hamilton strata; biostratinomic data indicate two very different enrollment postures. Interlocking morphologies (coaptative devices) and apodeme structure and disposition indicate that these postures reflect specific behaviors which involved interaction between tergal structures, inferred musculature, and the substratum.Phacopsenrolled by burrowing forward and down into the sediment; dorsal muscles, attached to prominent articulating half‐rings, imbricated the thorax such that each lappet overlapped the next posterior segment and locked into a posterior pleural facet. The pygidium was brought into place as the posterior segments of the thorax were placed into vincular notches along the lateral margin of the ventral cephalon. The pygidium locked with the cephalic vincular furrow to complete ‘perfect sphaeroidal’ closure.Greenopsenrolled with the cephalon in an upright position at the sediment surface; a submarginal furrow on the ventral surface of the pygidium received the anterior rim of the cephalon. Relatively narrow articulating half‐rings limited pleural rotation. Segments were loosely locked into narrow facets at the anterior margin of the next posterior lappet. In spite of rudimentary lappet and half‐ring structures,Greenopsdisplays an elaborate system of thoracopygidial muscles which involved dorsoventral and longitudinal attachments along the thorax and into the pygidium.Phacops, in contrast, displays very poorly developed apodemes which occur in the thorax only. Longitudinal muscle strength was likely less important duringPhacopsenrollment than is evident for theGreenopsenrollment procedure. Conversely,Phacopsclearly relied to a great degree upon competent closure devices which are poorly developed inGreenops.Biostratinomic data reveal different enrollment behaviors which reflect the function of different enrollment‐related morphologies present in e

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb02063.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

A new trace fossil,Raaschichnus gundersoni, from the Upper Cambrian St. Lawrence Formation of Wisconsin, was produced by an aglaspidid arthropod. The rusophyciform trace, which occurs singly and in series, is distinctive in showing marks left by the tail‐spine. Other trace fossils previously considered to have been made by aglaspidids were probably excavated by animals lacking a tail‐spine and with appendage morphologies very different from the aglaspidids of Wiscon

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb02064.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

After reaching a diversity peak in the Caradocian, North American Ordovician crinoids underwent a gradual decline to a nadir in the early Ashgillian (Maysvillian). This interval, recording extinction of the Cleiocrinidae, Merocrinidae, Ottawacrinidae, Hybocystitidae, and several lineages of camerate crinoids, was apparently caused by major environmental shifts in seas of eastern North America resulting from a westward‐prograding wedge of terrigenous clastics derived from the Taconic Highlands, possibly coupled with a marine transgression in the Maysvillian that allowed colder water slope biofacies to invade the craton. Crinoids suffered a major episode of extinction in the late Ashgillian (late Richmondian/Rawtheyan). This event, preceding the end of the Ordovician by at least one stage or 2 to 4 million years, resulted in extinction of 12 families of crinoids including the Xenocrinidae, Tanaocrinidae, Reteocrinidae, Archaeocrinidae, Anthracocrinidae, Cincinnaticrinidae, Iocrinidae, Anomalocrinidae, Carabocrinidae, Cupulocrinidae, Porocrinidae, and Hybocrinidae. Glacio‐eustatic lowering of sea level may have triggered this crisis by partially draining the North American craton, resulting in changes in oceanic circulation, salinity, and temperature. Latest Ordovician (Hirnantian) carbonates of the North American mid‐continent region contain pelmatozoan assemblages from which Silurian crinoids radiated. These taxa were largely unaffected by a minor extinction event at the Ordovician/Silurian bou

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb02065.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb02066.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Synarthrial fulcral ridges are found in crinoid columnals from the mid‐Ordovician to the present and in all four subclasses. Similar articulations did not become common in the cirri until the Mesozoic. Synarthrial stem articulations fall into two broad groups. Type I articulations have a fulcral ridge in the centre of the articular facet. In elliptical ossicles this ridge corresponds to either the long (IA) or short (IB) axis of the facet. Although both are functionally similar, Type IA ossicles are more common. Type II articulations have an excentric fulcral ridge, parallel to either the long (IIA) or short (IIB) axis of the articular facet. Type IIA articulations are found in crinoid stems capable of coiling. Type II articulations are particularly common in the cirri of articulates and are well adapted for attachment to hard and soft substrates. Columnals with Type I articulations often have divergent fulcra, giving the stem flexibility in all directions, but this feature is not seen in cirri or in coiled stems, where it would impair normal functions. Only the cirri of isocrinids and comatulids are muscular, so the movement of columns with fulcra must be passiv

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb02067.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb02068.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

Three types of growth lines are recognised on articulate brachiopod shells: (1) very fine diurnal growth lines formed by calcite increments at the shell margin, (2) seasonal growth lines, formed by inward reflection (doubling back) of the mantle edge, seen as concentric steps on the shell surface and marked by re‐orientation of growth vectors evidenced by secondary shell fibres, (3) disturbance lines, formed by abrupt regression of the mantle edge, also seen as concentric steps on the shell surface, but indicated by a dislocation in the shell fabric. Lamellose and spinose ornaments of the sort seen inTegulorhynchiaare essentially genetically controlled. Periodic outgrowths from the outer mantle lobe secrete frills of primary shell that project from the shell surface and form short hollow spines where they cross the radial ornament. In longitudinal section spine formation is seen to involve gradual increase in the rate of secretion of primary shell followed by retraction, and often collapse, of the mantle outgrowth, accompanied by regression. Reflection of the mantle edge usually follows spine formatio

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb02069.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

ISSN:0024-1164    

DOI:10.1111/j.1502-3931.1988.tb02070.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY